1. Field of the Invention
The invention relates to certain cyclic peroxyketals useful as cross-linking/blowing agents and, in particular, to their use in compositions and processes for the preparation of foamed, cross-linked polyethylene.
2. Prior Art
It is known to form foamed, cross-linked polyethylene using chemical blowing agents by heating an expandable mixture comprising a thermoplastic resin such as polyethylene, a cross-linking agent and a chemical blowing agent. U.S. Pat. No. 3,470,119 teaches a method for manufacturing foamed polyethylene by use of a chemical blowing agent. U.S. Pat. No. 3,658,730 teaches a specific composition for blowing thermoplastic resins.
Chemical blowing agents (CBAs) are generally solid compounds or compositions which decompose at a specific temperature to yield a large volume of gas and a solid decomposition residue. Sodium bicarbonate, the most widely used inorganic CBA has limited use because it decomposes over a broad range of temperatures and because its decomposition point occurs at temperatures below those at which most resins, including polyethylene, are processed.
Organic CBA's tend to evolve gas over a more defined temperature range. Modern Plastics Encyclopedia 1979-1980, p. 186-188 teaches a group of known organic CBAs useful in the production of foamed, cross-linked polyethylene. The blowing composition of U.S. Pat. No. 3,658,731 utilizes azodicarbonamide, a known organic blowing agent, in conjunction with a chromium compound. The use of azobisformamide, p,p'-oxy-bis(benzenesulfonyl hydrazide), diazoaminebenzene dinitrosopentamethylenetetramine, 4-nitro-benzene sulfonic acid hydrazine, .beta.-naphthalene sulfonic acid hydrazide, diphenyl-4,4'-disulfonyl azide, and barium azodicarboxylate as blowing agents is taught in U.S. Pat. No. 3,470,119. A disadvantage with such azo blowing agents is the production of small amounts of ammonia which can corrode equipment.
The production of crosslinked polyethylene foams generally requires the use of cross-linking agents in addition to the blowing agent. An exception to the requirement of separate cross-linking and blowing agents is disclosed in U.S. Pat. No. 4,129,531 and U.S. Pat. No. 4,101,464 wherein certain azo esters function as cross-linking/blowing agents. However, as with other azo blowing compounds, small amounts of ammonia are produced.
Many organic peroxides, including cyclic peroxyketals, are known to be effective cross-linking agents for thermoplastic resins. U.S. Application Ser. No. 688,874, filed May 21, 1976 teaches nine-membered peroxyketals useful as cross-linking agents for polyethylene.